1. Field of the Invention
The present invention relates to an optical scanning device for producing electrical picture-information signals representative of a scene under surveillance. In particular, but not exclusively, the invention relates to such a device adapted for producing images from light in the infra-red region of the spectrum.
When used in this specification the terms "optical" and "light" will be understood to relate to electromagnetic radiation which extends beyond the visible spectrum and includes, inter alia, infra-red radiation.
2. Description of the Prior Art
Imaging devices incorporating optical scanners are known as such. These devices include a telescope directing incident light onto the facets of a polygonal reflector which is driven to rotate so that reflected light is scanned onto a plane reflector (perhaps after one or more further beam folding reflections at plane or curved reflectors) which is caused to oscillate about a given axis in order to change the elevation of the scanned beam orthogonally with respect to the scanning introduced by the rotating polygon. The light from the oscillating plane reflector is directed onto a detector array comprising a plurality of rows of photosensitive elments which produce electrical output signals representing the light incident thereon.
The displacement of the beam in the optical scanner introduced by the rotating polygonal reflector is termed the azimuth scan and the displacement within the optical scanner caused by the oscillating plane reflector is called the elevation scan. The azimuth scan rate is determined by the speed or rotation of the rotating polygonal reflector and, other things being equal, this speed will be related to the number of detector elements perpendicular to the azimuth scan, encompassed in a scan band. The wider the scan band the slower the polygonal reflector has to rotate. This means that for a given speed of rotation of the polygonal reflector there is an upper limit on the width of the scan bands (it being important in some circumstances to maintain the speed or rotation of the polygonal reflector relatively high) and since there are in practice other constraints which limit the number of detector elements in each row there is therefore an upper limit on the total number of detector elements in the detector array, which imposes a limit on the resolution of the eventual image produced by the imaging device of which the optical scanner forms part.
Although prior art devices employ polygonal reflectors it would be possible alternatively to employ refractors and references hereinafter to "optical elements" will be understood to refer to refractors or reflectors.
The technical problem of the present invention is thus to seek means by which, without having to reduce the speed of rotation of an optical element, such as a polygonal reflector in a scanning optical system of an imaging device, it will nevertheless be possible to increase the number of detector elements in a scan band beyond the limit which has previously been imposed by the constraint mentioned above. The present invention seeks to do this in such a way that the signal-to-noise ratio of the scanner is also improved.